Oxidative alkoxycarbonylation of olefins, such as the reaction of ethylene or propylene with carbon monoxide and an alcohol in the presence of a palladium(II) compound and an inorganic oxidizing agent, is disclosed in U.S. Pat. Nos. 3,397,225, 3,397,226, 3,530,168, 3,876,694 and 3,907,882. It is believed that in these reactions, the palladium(II) is reduced to palladium(O) and, in a single cycle, oxidized by the inorganic oxidizing agent. The inorganic oxidizing agent can be oxidized with molecular oxygen. A problem with such processes is that the inorganic oxidizing agent, typically a copper halide, may be highly corrosive.
In a catalyst system involving a precious metal like palladium, it is desirable to achieve high catalyst efficiency to permit use of a low concentration of the precious metal. One measure of catalyst efficiency is "palladium turnover", i.e., moles of useful products per gram-atom of palladium in the catalyst. By "useful products" is meant those products, in the oxidative alkoxycarbonylation of olefins, which are commercially desirable, i.e., other than undesirable byproducts. It is apparent that characterization of products as useful depends upon the particular needs of those performing the reaction. It is further apparent that the carbon content and structure of the useful products will vary depending upon the reactants. When the olefin subjected to oxidative alkoxycarbonylation in the presence of an appropriate catalyst is a butene, and the other reactants are methanol, carbon monoxide, and oxygen, products which for purposes of this disclosure are characterized as useful products are methyl pentenoates and methyl 3-methoxy-pentanoate, hereinafter referred to as methyl .beta.-methoxyvalerate. Methyl .beta.-methoxyvalerate is readily converted to methyl 2-pentenoate in a separate process by the method of Weizmann et al., j. Am. Chem. Soc., 70, 1153 (1948). Methyl pentenoates are characterized as useful because they can be converted by further carbonylation in a separate process to dimethyl adipate which is readily converted to adipic acid for the production of polyamides.
It has been found that use of a palladium/heteropoly acid catalyst system for the oxidative alkoxycarbonylation of olefins allows achievement of high palladium turnover with a noncorrosive catalyst system.
It is an object of this invention to provide a catalytic process for the oxidative alkoxycarbonylation of olefins with a palladium/heteropoly acid catalyst system.
It is a specific object of this invention to provide a palladium/heteropoly acid catalyzed process for the oxidative alkoxycarbonylation of 1-butene to methyl pentenoates and/or methyl .beta.-methoxyvalerate, which process can be carried out using a low concentration of palladium. Other objects will become apparent hereinafter.